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# 化学代写|有机化学代考Organic Chemistry代写|CHEM251 Seeing Molecular Symmetry

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## 化学代写|有机化学代考Organic Chemistry代写|Seeing Molecular Symmetry

NMR spectroscopy reports on the different chemical environments of the nuclei. You may expect every nucleus in a molecule to give rise to a different signal in the NMR spectrum. In this case, a molecule with five hydrogens would give rise to five signals in the ${ }^{1} \mathrm{H}$ NMR spectrum, and a molecule with six hydrogens would give rise to six signals, and so forth. This is sometimes the case, but as far as NMR is concerned, nuclei that are in identical chemical environments are indistinguishable and give rise to only one signal.

Identical chemical environments means two or more nuclei are attached to exactly the same types of atoms in the same order. The simplest example is $\mathrm{H}_{2}$, in which both hydrogens are in identical chemical environments and both are attached to one $\mathrm{H}$. A more common example is that, in general, two hydrogens attached to the same carbon are in identical chemical environments. Two (or more) nuclei that exist in identical chemical environments give rise to a single peak in the NMR spectrum, making these nuclei chemically equivalent.

A lot of times, chemical equivalence arises because of molecular symmetry. In a molecule that has a plane of symmetry, one side of the molecule is the mirror image of the other side, and any atom on one side of the symmetry plane has a twin on the other side of the plane in exactly the same chemical environment. These two chemically equivalent atoms show up as a single peak in the NMR spectrum.

## 化学代写|有机化学代考Organic Chemistry代写|Working with Chemical Shifts, Integration, and Coupling

In this section, you practice working with the individual parts of an NMR spectrum:
s Chemical shift: This is where the peaks show up on a spectrum. The higher the chemical shift of a peak (larger ppm value), the more deshielded that nucleus is. In most cases, deshielding is caused by adjacent electronegative atoms such as $\mathrm{O}, \mathrm{N}$, and halogens. Benzene ring hydrogens are also deshielded by the magnetic field caused by the circulating electrons in the aromatic ring, which is termed diamagnetic anisotropy for those of you who like naming things (scary word, though, huh?).

Integration: Integration tells you the area under each peak, which is related to how many protons a peak represents. A peak that integrates for twice that of another peak (in other words, it has twice the area underneath the peak) means that that peak represents twice as many protons. Sometimes the area is indicated by $s$-shaped curves over the peaks. The height of these $s$-shaped curves gives information about the relative number of hydrogens in that peak.
Coupling: Coupling is the way some peaks split into a number of different lines. The number of lines a peak splits into depends on the number of hydrogen neighbors the peak has, following the $n+1$ rule. Thus, a proton with one hydrogen neighbor splits into two lines (a doublet), a proton with two hydrogen neighbors splits into three lines (a triplet), and so forth.

## 化学代写|有机化学代考Organic Chemistry代写|Working with Chemical Shifts, Integration, and Coupling

s 化学位移：这是峰出现在光谱上的位置。峰的化学位移越高（ppm 值越大），核的屏蔽越多。在大多数情况下，去屏蔽是由相邻的电负性原子引起的，例如○,ñ, 和卤素。苯环氢也被芳香环中循环电子引起的磁场所屏蔽，对于那些喜欢命名事物的人来说，这被称为抗磁各向异性（虽然是个可怕的词，嗯？）。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。